1. Field of the Invention
This invention relates to a process for preparing malt, in which barley is moistened and kept at a germinating temperature until germinated barley is obtained, after which the germinated barley is dried and heated (kilned) in a stream of heated air.
2. Discussion of the Art
Some time ago it was discovered that nitrosamines may, under certain conditions, have a carcinogenic effect. Nitrosamines may occur in beer, and many countries require its content of nitrosamines to be below a very low limit.
It was known that these nitrosamines are not present in the malt in appreciable quantities before the kilning process, but are formed therein during kilning, whence they find their way into the beer.
In the majority of conventional kilning processes, the very moist germinated barley (circa 45% water) is brought into direct contact with a stream of heated gas, whereby the germinated barley is first dried and then heated at a temperature at which the enzymes are largely preserved, and growth is stopped. The removal of the moisture requires a continuous stream of air of a moderately high temperature, and this air is commonly produced by mixing combustion gases with secondary air.
During the combustion of the fuel used, owing to the high flame temperature an amount of nitrogenous oxides is formed, and these are carried along with the stream of gas to the kiln. The germinated barley always contains amino groups, which then react with the nitrogenous oxides to ultimately form the nitrosamines. In vitro the reaction only proceeds to an appreciable extent in a narrow pH range.
In an alternative kilning process the barley is heated indirectly by hot gases, while a stream of secondary air is passed through the barley. In that case nitrogenous oxides from the combustion gases cannot of course react with the barley. Even so, nitrosamines are often produced in the malt, as the air passed through the barley is withdrawn from the atmosphere, which especially in industrial areas and densely-populated areas contains appreciable proportions of nitrogenous oxides, which lead to the formation of nitrosamines. Furthermore, a kilning process with indirect heating has the disadvantage of requiring considerably more expensive equipment, while the total energy consumption is approximately 5% higher.
Up until several years ago, the fuel used in the kilning process was coal or a petroleum product. These fuels virtually all have a considerable sulphur content.
From environmental considerations and urged by authorities, maltsters then changed over to the use of low-sulphur fuels for the kilning process, such as natural gas, which is virtually free of sulphur.
It then turned out that kilning with a low-sulphur fuel resulted in the proportion of nitrosamines in the resulting malt and the beer prepared from it being much higher than when sulphur containing fuels were used.
For this reason the kilning process is at present often carried out using natural gas as the fuel and adding sulphur dioxide to the stream of air, for example, by combusting sulphur. This may indeed result in a low content of nitrosamines, but is only successful if a certain concentration of sulphur dioxide is maintained during the first kilning phase.
A conventional value for the content of sulphur dioxide is 1000 g/ton air-dry barley to be steeped. If the supply of sulphur dioxide is interrupted, nitrosamines will still be formed, and these are not decomposed when sulphur dioxide is again supplied.
The disadvantages of that method are, therefore, as follows:
(a) kilning becomes more complicated and requires more intensive and more accurate supervision;
(b) there is yet an effluent of sulphur dioxide, which is undesirable from environmental considerations, and may well be more hazardous than the nitrosamines themselves;
(c) sulphur dioxide may give rise to the corrosion of kilning equipment.
Up until now, the maltster has been unable to control the nitrosamine level without using sulphur dioxide, as he is dependent on the proportions of nitrogenous oxides in the kilning air drawn in.